Oxygen sensors have received widespread attention particularly for applications like combustion control, process control, and medical applications. In the automotive area, oxygen sensors are used to control the Air-to-Fuel Ratio (A/F) of internal combustion engines. The great majority of present day automobiles employ an electrochemical-type oxygen sensor to control the A/F ratio. Generally this ratio is controlled at the stoichiometric value, about 14.4-14.7, where the so-called three-way catalysts have the greatest efficiency for eliminating regulated emissions (hydrocarbons, carbon monoxide, and oxides of nitrogen) from the exhaust gas. The automotive oxygen sensor, sometimes called a lambda sensor, includes an oxygen-ion-conducting solid electrolyte, generally ZrO.sub.2, in the form of a thimble with porous platinum electrodes deposited on the outside and the inside surfaces of the thimble. The inside of the thimble is exposed to ambient air as a reference atmosphere, whereas the outside of the thimble is exposed to the exhaust gas. When the thimble is heated (e.g. temperatures higher than 300.degree. C.) and there is a difference in the oxygen partial pressures P.sub.O2 between the two sides of the ZrO.sub.2 thimble, an electromotive force (emf) is generated between the two Pt electrodes with a value given by the Nernst equation: emf=(RT/kT) ln(P.sub.O2,exh/PO.sub.2,air).
When the sensor is placed in the exhaust gas of a vehicle, and the A/F is varied, the sensor emf shows a large and abrupt change at the stoichiometric A/F value as shown in FIG. 1. The reason is that the thermodynamic equilibrium oxygen partial pressure in the exhaust gas changes by many orders of magnitude at the stoichiometric A/F ratio. Away from stoichiometry, the emf varies only slowly with A/F because the partial oxygen pressure also changes only slowly with A/F.
Another type of a high temperature oxygen sensor useful in automotive applications is a resistive-type sensor based on TiO.sub.2. At elevated temperatures, the electrical resistivity of TiO.sub.2 and of many other metal oxides such as SrTiO.sub.3 and CoO depends on the oxygen partial pressure P.sub.O2 in the ambient gas atmosphere. This dependence is generally weak, e.g. for TiO.sub.3, SrTiO.sub.3 and CoO this dependence is a positive or negative 1/4 to 1/6 power dependence. In spite of their weak P.sub.O2 sensitivity, TiO.sub.2 sensors are useful for stoichiometric A/F control because they also exhibit a large and abrupt resistance change at the stoichiometric A/F value as the result of the large change in exhaust gas P.sub.O2 near stoichiometry.
FIG. 3 shows results of measurements of the resistance of a porous cobalt oxide ceramic as a function of P.sub.O2 at several temperatures. The large stepwise change in the resistance of the sample at temperatures below 1000.degree. C. is associated with the phase change from CoO to Co.sub.3 O.sub.4. The resistivity of Co.sub.3 O.sub.4 is independent of P.sub.O2, whereas the resistivity of CoO decreases with increasing P.sub.O2 according to the relationship R=A exp(E/kT) (P.sub.O2).sup.-1/4. This property of CoO has been utilized in fabricating oxygen sensors as disclosed by G. L. Beaudoin et al., SAE Paper #760312, Feb. 23, 1976 and U.S. Pat. No. 3,933,028 to K. R. Laud et al. The oxygen sensitivity of this sensor is, however, low.
There is a growing need for oxygen sensors that can measure A/F away from stoichiometry, in particular in the lean A/F region where excess oxygen is employed. These A/F ratios are often 19-40. Many engines are being modified for lean operation because of the fuel economy advantages which can be achieved. These sensors must have high P.sub.O2 sensitivity because the oxygen partial pressure in the exhaust gas does not change appreciably with A/F in this region. The ZrO.sub.2 lambda sensor and the resistive-type sensors mentioned above, have very limited sensing ability away from stoichiometry. In addition the resistive-type sensor generally have a strong dependence on temperature.
On the other hand, another type of sensor, i.e., ZrO.sub.2 based sensors operating in the oxygen pumping mode have much higher sensitivity (e.g. 1st-power P.sub.O2 dependence) away from stoichiometry. Examples of this type of sensor are the Universal Exhaust Gas Oxygen Sensor (UEGO) and the Lean Exhaust Gas Oxygen Sensor (LEGO) based on two ZrO.sub.2 cells. FIG. 4 shows the output of the UEGO sensor as a function of .lambda.=(A/F)/(A/F).sub.stoic, the A/F normalized to the stoichiometric A/F value, (A/F).sub.stoic. These sensors are complex, however, and consequently expensive to manufacture which Limits their widespread commercialization. Consequently, effort has been expended to develop a simple sensor having high oxygen sensitivity for A/F measurement away from stoichiometry.
Another type of sensor for monitoring the oxygen content in oxygen rich exhaust gases from furnaces is disclosed in U.S. Pat. No. 4,351,182 to Schmidberger. The oxygen sensitive element is a palladium layer maintained at a specific high temperature, e.g., 700.degree. C., whereby the palladium metal (Pd) changes phase to palladium oxide (PdO) when the oxygen partial pressure exceeds a specific "critical" value, P.sub.O2,c. It is disclosed that the phase change from Pd to PdO and vice versa causes a change in the electrical conductivity by a factor of about 20 which is used to provide a sensor output signal. This is shown in FIG. 5 where the percent change in the conductivity of Pd/PdO is plotted as a function of the .lambda. for two different temperatures. It is also known that many metal oxides change to other metal oxide phases when the temperature or the P.sub.O2 are changed appropriately. For example, at a given temperature, CoO is stable at low P.sub.O2, but at higher P.sub.O2, it transforms to Co.sub.3 O.sub.4 as shown in FIG. 2. Such metal-oxide to metal-oxide phase transitions are also generally accompanied by large changes in electrical resistivity as shown in FIG. 3 for cobalt oxide. Similarly, another physical property, optical absorption, in the case of phase transformations for Sr-Fe-oxide, shows a large associated change.
Such phase transitions, however, are known to be generally accompanied by significant hysteresis unless the P.sub.O2 is changed to a value substantially larger (or smaller) than P.sub.O2,c. Hysteresis is related to the time that it takes for the material to change from one phase to another and is believed to be associated with a necessary nucleation process, i.e., formation of critical size nuclei of the new phase. The presence of hysteresis in phase transformation type sensors would negatively impact their usefulness since they would have relatively long response time. In copending application entitled "Metal Oxide Oxygen Sensors Based on Phase Transformations" mentioned above, we disclose oxygen sensors based on such phase changes with do not suffer from the problems associated with nucleation processes. These sensors generally comprise sensing material across which is provided a temperature gradient to maintain the sensing material in at least two phases with a boundary line therebetween. The movement of the boundary line across a fixed detecting location on the material is used to indicate a change in oxygen partial pressure of the ambient atmosphere to which the sensor is exposed.
Oxygen or A/F sensors which exhibit a relatively abrupt and large change in output, e.g., related to change in resistivity of the material, at some specific "critical" value of the oxygen partial pressure P.sub.O2,c or (A/F).sub.c can conveniently provide information only on whether the ambient PC.sub.O2 (or A/F) is lower or higher than P.sub.O2,c or (A/F).sub.c. When these sensors are used to feedback control the ambient P.sub.O2 or (A/F) at some specific value P.sub.O2,c or (A/F).sub.c, the feedback control is of the "limit-cycle" type rather than proportional control. For example, in the limit-cycle control of A/F of an engine, the A/F ratio is ramped from a value lower than (A/F).sub.c to a value higher than (A/F).sub.c or vice versa, with the direction of the ramping depending on the sensor output. When A/F passes through (A/F).sub.c, the sensor signal changes from a low to a high value or from a high to a low value and the electronic feedback system is ordered to change direction of the A/F sweep. Consequently, the A/F oscillates between two A/F values, one lower and the other higher than (A/F).sub.c, at a certain frequency called "limit-cycle frequency". This is the basic method by which the ZrO.sub.2 lambda sensor is used extensively for stoichiometric A/F control of present day vehicles and such limit-cycle control is the principle of sensing and controlling of the copending application invention mentioned above.
However, it has been found that a proportional A/F control rather than a limit-cycle control can lead to better performance of vehicles with respect to tailpipe emissions. In this respect, the more complex and costly O.sub.2 -pumping-based ZrO.sub.2 sensors (e.g., the UEGC sensor) with their approximately linear response to A/F ratio are preferred over the Nernst-type ZrO.sub.2 lambda sensor for stoichiometric A/F control. The same O.sub.2 -pumping-based A/F sensors can also be used for proportional A/F control away from stoichiometry, e.g., in the lean.
The simpler and less expensive resistive-type sensors of the prior art such as the CoO sensor are applicable for proportional A/F control in the lean, but their accuracy is limited because of their low P.sub.O2 sensitivity, as discussed previously. The more sensitive resistive sensors of the prior art with a sharp and abrupt change in their response at some lean A/F value (e.g., the Pd/PdO sensor of U.S. Pat. No. 4,351,182) obviously cannot be used for proportional control. The sensor of noted copending application "Metal Oxide Oxygen Sensors Based on Phase Transformations" also shows a relatively abrupt and large change in output, e.g., related to the electrical resistance change of the sensing material at some specific value of the oxygen partial pressure of A/F, and hence can not be used for proportional control. The response of the sensor is shown in FIG. 6. In this example the sensor is designed to detect a P.sub.O2 of 0.031 atm. For comparison, the response of the prior art CoO resistive sensor is also shown in FIG. 6.
Because of the disadvantages of the O.sub.2 -pumping-based electrochemical Zr.sub.O2, it is still highly desirable to develop A/F sensors, e.g. resistive-type sensors which have a non-abrupt response but also are very sensitive, especially in the lean A/F region. This is an object of the present invention oxygen sensor. The present invention overcomes the deficiencies of prior art sensors and provides an oxygen sensor useful for automotive applications having excellent sensitivity and response time in a variety of A/F ratios, including that of lean-burn gasoline or diesel engines.